Modern ceramics include a wide range of materials ranging from single crystals and dense polycrystalline materials through glass-bonded aggregates, insulating foams and wholly vitreous substances. As the need for ceramic materials has grown, so has the desire to control their nucleation and growth, as this will frequently determine the useable properties of the final product.
One of the recent methods for controlling the nucleation and growth of inorganic and organic crystals is the use of supramolecular assemblies composed of organic molecules, which serve as templates, for forming the ceramic or other crystalline structure. This method of control has a direct influence on crystal location, polymorph selectivity, and the microstructure of the crystals formed.
The use of templates also occurs in nature. For example, aragonite is one of a number of calcium carbonate (CaCO.sub.3) polymorphs found in nature (e.g., in mollusk shells, human brain stones, gallstones, and the Earth's crust). In mollusk shells, CaCO.sub.3 mineralization occurs in an insoluble organic matrix which acts as a template for incipient crystallization. X-ray diffraction analysis has suggested that the organic template assumes antiparallel .beta.-sheet conformation. More specifically, a close match has been found between the matrix periodicity and Ca--Ca distances in the ab plane of aragonite, particularly along the a axes (4.96 .ANG. and 4.75 .ANG., respectively) and less of a match along the b axes (7.97 .ANG. and 6.9 .ANG., respectively). S. Weiner and W. Traub, X-Ray Diffraction Study of the Insoluble Organic Matrix of Mollusk Shells, FEBS Letters, Vol. 111, p. 311 (1980).
The synthesis of aragonite has been extensively studied and has been achieved mainly through the introduction of various inorganic and organic additives to calcium-containing solutions and gels. For example, aragonite has been formed in the presence of NaCl and hexametaphosphate or pyrophosphate. Metastable formation of aragonite in gels has been achieved at elevated pressure in a temperature range of 100.degree. to 270.degree. C. in the presence of magnesium ions.
Additives such as Mg, Ni, Co, Fe, Zn, Cu and Li may lead to aragonite growth at ambient conditions, i.e., room temperature and atmospheric pressure. For example, in M. Okumura and Y. Kitano, Coprecipitation of Alkali Metal Ions with Calcium Carbonate, Geochemica et Cosmochimica Acta, Vol. 50, pp. 49-58 (1986), the authors describe the use of alkali metal ions (Li.sup.+, Na.sup.+, K.sup.+, and Rb.sup.+) and Mg.sup.2+ ions dissolved in a calcium bicarbonate solution. The solution was stirred at 25.degree. C., until 80-90% of the calcium ions in the solution were precipitated as calcium carbonate, by degassing CO.sub.2 from the solution. The calcium carbonate was filtered off, washed and air dried. The crystal form of the calcium carbonate precipitated from the parent solutions was found to be pure aragonite with a small amount of magnesium ions, and pure calcite without magnesium ions. Similarly, in B. Heywood and S. Mann, Molecular Construction of Oriented Inorganic Materials, Chemical Materials, Vol. 6, pp. 311-318 (1994), the authors state that the precipitation of aragonite is favored by the presence of Mg.sup.2+ ions in the crystallization medium.
Litvin et al. have demonstrated the influence of supramolecular diacetylenic template structures on spatial location and morphology of CaCO.sub.3 crystals. Influence of Supramolecular Template Organization on Mineralization, Journal of Physical Chemistry, Vol. 99, No. 32, pp. 12065-12068 (1995). There, calcium carbonate crystals were grown from a calcium bicarbonate solution in the presence of stearic acid, diacetylene modified by glycine, and hydroxyl ethylamine. The crystal growth occurred under the liquid crystalline template, which was manipulated to control the density of nucleation sites and the morphology of the crystals. The periodic modulations in the template influenced the crystal growth locations, and the local density of the template influenced the polymorph selection.
It is also known to precipitate calcite and aragonite by mixing soluble carbonate solutions containing stronium, barium, or lead with solutions of calcium ions. Pure aragonite was precipitated at temperatures of 50.degree. C. and above. Wray and Daniels, Precipitation of Calcite and Aragonite, Journal of the American Chemical Society, Vol. 79, No. 9, pp. 2031-2034 (1957).
Recently, a method has been described for synthesizing hollow porous shells of aragonite that resemble the coccospheres of certain marine algae. In this method, aragonite is prepared from a bicontinuous microemulsion containing a cationic quaternary ammonium surfactant (didodecyldimethyl ammonium bromide), tetradecane, supersaturated calcium bicarbonate solution and Mg.sup.2+ ions. D. Walsh and S. Mann, Nature, Vol. 377, 320 (1995).
Biomineralization studies have been utilized to analyze the controlled fabrication of synthetic materials, such as templated crystals. In Fritz et al., Flat Pearls From Biofabrication of Organized Composites In Inorganic Substrates, Letters to Nature, Vol. 371, pp. 49-51 (1994), highly-organized aragonitic nacre--a flat pearl--was biofabricated on disks of glass, mica or MoS.sub.2 inserted between the mantle and shell of Haliotis rufescens (red abalone). Once a partially ordered calcite layer had been deposited, there was a switch back to the nucleation and assembly of stacks of highly-ordered aragonitic nacre. The presence of an inorganic surface between the mantle and shell, therefore, triggers a change in the nature of the mineral phase.
Most of the competitive methods of aragonite manufacture require the use of high temperature, high pressure, specific additives or other difficult procedures, and even with these methods, the degree of control over inorganic phases is limited. This leads to high processing costs and often results in brittle failure of the product in use due to imperfections or voids left during the nonhomogeneous processing.
As described above, many researchers have studied the process of ceramic growth using thin films or self-assembled monolayers; however, no reports have demonstrated adequate control over crystal morphology. Some researchers have added co-precipitants in order to improve control. Other studies show that oriented nucleation of either calcite or vaterite is dependent on the structural and chemical properties of the monomolecular film (i.e., a film having a thickness the length of one molecule). However, none of the surface active agents (surfactants) studied to date selectively induced nucleation of aragonite unless additives, such as Mg.sup.2+ ions, were introduced into the supersaturated subphase. Also, all surfactants previously used for mineralization have involved a pseudohexagonal packing in the solid phase. It is important, therefore, to create alternative synthetic pathways for the production of aragonite.
Another class of ceramics which have been extensively studied are the perovskites. Their high electrical and optical properties enable their use in thermistors, high-dielectric capacitors, field-effect transistors, nonvolatile memorys, and high-frequency transducers in the form of a thin film. The perovskite structure is typically in the form: XYO.sub.3, with the X atom surrounded by 12 O atoms and the Y atom by 6 O atoms. This structure provides a strong ferroelectric material with a strong electric-dipole moment and therefore enables storage of large quantities of electric energy similar to a capacitor. The material has a large dielectric constant due to the central ion's ability to move within the structure, causing electric poles. Representative perovskite materials include barium titanate (BaTiO.sub.3), strontium titanate (SrTiO.sub.3), and doped combinations of these materials, collectively known as BSTO's. The ability to control the microstructure, and hence, the physical properties of these materials is of significant importance in the electronics industry. Other structural-dependent classes of materials having the perovskite structure include antiferroelectric materials (PbZrO.sub.3 and NaNbO.sub.3), ferromagnetic compounds (LaCo.sub.0.2 Mn.sub.0.8 O.sub.3), and anti-ferromagnetic compounds (LaFeO.sub.3).